User's Guide

The Operational View SB300 Series
Page 26 Proprietary and Confidential 2110059 Rev 1.0
The more advanced protocols, such as V.34 for example, are smarter in that they are capable of
simultaneously supporting different rates on the transmit and receive lines, and allow two modems
to exchange information to fine-tune their modulation parameters to optimize performance.
4.5.1.2. Error Control
The next layer is the Error Control Protocol Layer which implements one of a number of error
encoding and detection schemes on outgoing and incoming data frames respectively.
By appending extra encoding bits to outgoing data packets, it allows the receiving modem using
the same Error Control protocol to check whether the data has been corrupted during transmission
and demodulation by the lower layer. Similarly, it processes data frames received from the other
modem by using the appended encoding information to check for corrupted data.
In addition to error coding and detection, this layer also takes care of the retransmission of
corrupted packets to ensure that data delivered to the layer above it is error free.
This layer is also provided in several standards: LAPM, MNP2 through 4, MNP10, etc.
4.5.1.3. Data Compression
The third layer is the Data Compression and Decompression Protocol. On outgoing data, this
layer effectively reduces (or compresses) the number of bits coming from the application layer
above it to a smaller number of bits without losing any information. The receiving modem then
decompresses this data and reconstructs it to its original form.
Considering that the slowest link between two modems is always the wireline or cellular link, this
scheme reduces the amount of data flowing on that link and consequently increases the overall
data throughput.
This layer is implemented by a variety of protocols, two of the most widely used ones being
V.42bis and MNP5.
4.5.2. CSC vs. Wireline
Older modem protocols, such as V.32 and earlier, were developed and optimized for PSTN
wireline networks. However, with the advent of cellular phone networks, and the need for
wireless modems to operate within these networks, it became apparent that some adjustments had
to be made to the existing protocols to enable them to achieve performance similar to their
wireline counterparts.
4.5.2.1. Enhanced Throughput Cellular (ETC)
ETC was developed to bring cellular link performance up to wireline levels. Although it does not
exist as a layer of its own, it interacts with various layers by adjusting some of their critical
parameters to compensate for network differences. It uses a number of techniques to improve the
robustness of the data link.
At the lowest layer, ETC adjusts the filter characteristics which modulate and demodulate the
carrier signal to compensate for differences between PSTN and cellular networks. At the Error
Correction layer, it enforces the use of small data packets so that retransmissions are short in the
event of data corruption, which will be more frequent in a wireless environment.
Because ETC does not exist as a separate layer in the stack shown above, but instead makes use of
existing protocols, it is possible for an ETC-enabled modem to communicate with a non-ETC
modem. Although it is preferred that both sides of the modem link support ETC, one of the
attributes is that even if only one side supports ETC, significant benefits will be gained.
Also, due to the nature of ETC making use of existing protocols, ETC imposes certain constraints
on which protocols must be present at both ends for a connection to be established. The required
protocols are LAPM for Error Correction and V.42bis for Data Compression. Without these on